fix render_scaled_region for Pillow 4.2

[will-moore]

This fixes this issue python-pillow/Pillow#2609 reported by @pwalczysko using pillow==4.2.1.

To test, add a Big Image to figure and check that it renders OK.

[pwalczysko]

This change works for me on the localhost mac os x, with pillow 4.2.1 (installed using conda)

[jburel]

Has this been tested with other version of Pillow?

[jburel]

Does the problem only occur when generating a jpg?

[will-moore]

There's nowhere else in views.py or Export script that we're saving PIL image as JPEG, so nowhere else needs fixing.
However, I made the same RGB change at views.py def default_thumbnail in case this changes to saving as JPG in future. Functionality is unchanged.

[jburel]

Thanks @will-moore